Culture and Sensitivity

Helping the Infection Preventionist Interpret Antibiograms

Walter Phillips, BS, MS, PhDDirector of MicrobiologyTriStar Healthcare System

Acknowledge

I would like to acknowledge the following contributors to this presentation:

Sheldon Campbell MD, PhD, FCAP Janet Hindler, MCLS MT(ASCP)

Objectives

At the conclusion of this presentation, the participant will be able to: Discuss the purpose of an

antibiogram. Recall the acronym CLSI. Recognize antibiograms for MRSA,

VRE, ESBL and CRE. Define MDRO.

What to Test

CLSI M-100 Sx (where x is an edition number)

Issued yearly, gives guidelines– Keep up!

Don’t report everything– Conform to facility

formulary– Report narrow-spectrum /

inexpensive agents routinely

– Reserve broad-spectrum / expensive agents for resistant isolates

CLSI M100-S24 Table 1A.

Group A

Group B

CLSI M100-S24. Table 1A..

Antimicrobial Susceptibility Testing

Can I use this drug to treat this bug?– With tolerable doses, will enough drug get

to the site of infection to kill the bug? MIC

– Minimal Inhibitory Concentration– broth-based, micro and macro methods

Disc Diffusion E-test

MIC Testing

Liquid media with varying dilutions of antibiotic

Lowest concentration that inhibits growth is the MIC– Minimal Inhibitory

Concentration Various formats available,

automated and non-automated

How to Report

Susceptible, Intermediate, Resistant (S,I, R)

S - means susceptible to usual doses in accessible sites It does NOT mean in the middle of a giant polymicrobial

abscess

I - means susceptible at increased doses or where the drug is concentrated

Usually means a drug to use in urine or if no better choice is available

R - means resistant to usual doses

The S, I, R MIC and disk-zone cutoffs are set by CLSI using epidemiological and pharmacodynamic data.

There may be additional value in some settings to report numerical MICs.

Enterobacteriaceae Cefepime MIC (µg/ml) Breakpoints

AgentOld Breakpoints New Breakpoints

Susc Int Res Susc SDD Res

Cefepime ≤8 16 ≥32 ≤2 4-8 ≥16

CLSI M100-S24. Table 2A.

(13) The interpretive criterion for susceptible is based on a dosage regimen of 1 g every 12 h.

The interpretive criterion for SDD is based on dosing regimens that result in higher cefepime exposure, either higher doses or more frequent doses or both, up to approved maximum dosing regimens.

SDD = “Susceptible Dose Dependent”

clindamycin ≤0.5 S erythromycin ≤0.5 Soxacillin ≤0.5 Spenicillin Rvancomycin ≤0.5 S

Specimen: Wound drainageDiagnosis: Trauma

Staphylococcus aureus

MIC (g/ml)

“Oxacillin-S and penicillin-R staphylococci are susceptible to other anti-staphylococcal ß-lactams (except amoxicillin, ampicillin, and penicillins).”Final Report with Optional

Comment

Specimen: BloodDiagnosis: Severe cellulitis

Staphylococcus aureus

ceftaroline 0.5 Sclindamycin >4 Rdaptomycin 0.5 Sdoxycycline ≤1 Serythromycin >4 Rlinezolid 1 S

oxacillin >8 R penicillin R

rifampin ≤0.5 Strimeth-sulfa ≤0.5/9.5 Svancomycin 2 S

“MRSA are resistant to all β-lactam agents (except ceftaroline). Doxycycline, rifampin or trimeth-sulfa should not be used alone for serious MRSA infections. Ceftaroline, daptomycin and linezolid reported per Dr. Jones request.”

Final Report with Optional Comment

MIC (g/ml)

Newer Antimicrobial Agents for MRSA - Daptomycin

RouteComments

PO IM IV

x

• FDA indications for complicated skin infections, bacteremia

• Disk diffusion does not work; must do MIC

• Susceptible breakpoint only• Resistance uncommon (<1%) but has been

reported• NOT for respiratory infections (lung

surfactant inhibits drug); do not report on respiratory specimens

Zone (mm) MIC (µg/ml)

S I R S I R

- - - ≤1 - -CLSI M100-S24. Table 2C.

Staphylococcus spp.

Newer Antimicrobial Agents for MRSA - Linezolid

RouteComments

PO IM IV

x x

• FDA indications for nosocomial pneumonia, complicated skin infections, uncomplicated skin infections (MSSA); community-acquired pneumonia (MSSA)

• For disk diffusion, examine zone with transmitted light

• Resistance uncommon (<1%) but has been reported

Zone (mm) MIC (µg/ml)

S I R S I R

≥21 - ≤20 ≤4 - ≥8CLSI M100-S24. Table 2C.

Staphylococcus spp.

New Antimicrobial Agents for MRSA - Ceftaroline

RouteComments

PO IM IV

x

• Broad-spectrum cephalosporin (with anti-MRSA activity)

• FDA indications for acute bacterial skin and skin structure infections; community-acquired bacterial pneumonia

• Resistance uncommon (<1%) but has been reported in MRSA

Zone (mm) MIC (µg/ml)

S I R S I R

≥24 21-23

≤20 ≤1 2 ≥4

CLSI M100-S24. Table 2C.

S. aureus

Glycopeptides and Lipopeptides

Major Drugs: Vancomycin, Teicoplanin, Daptomycin

Activity: Gram-positives Resistance Mechanisms: modification

of cell-wall target; increased cell-wall thickness with drug permeability changes, undescribed mechanisms for Daptomycin

Staphylococcus aureus

Major nosocomial and community-acquired pathogen– Responsible for >20% of bacteremia in US/Canada– Transmissible nosocomially and in the community.

Antibiotic resistance -- lots– Methicillin (oxacillin) resistance

Nosocomial Community-acquired

– Vancomycin resistance Relative resistance (VISA) High-level resistance (VRSA)

– Macrolide resistance The D-test

VISA

Vancomycin Intermediate S. aureus– MIC 4-8

Accumulated changes associated with decreased fitness– Thickened cell wall by EM– Mixed large & small colony

morphotypes on plates Associated with poor clinical response

VRSA

MIC ≥8 Acquisition of vanA cluster from

Enterococcus Typically very high MICs, no loss of

fitness– 1st Michigan strain: vanA cluster on a

Staph conjugal plasmid, MIC>1000– PA strain MIC=32 (?loss of cluster or

↓expression)– NY strain MIC=64– 2nd Michigan strain 04/05: MIC >256

VRE

Enterococci are naturally resistant to lots of important antibiotics– Cephalosporin's, SXT, clindamycin– Emerging resistance to penicillin's and vancomycin

Vancomycin resistant enterococci– Van A (E. faecium (most VRE isolates) and E. faecalis)

High level R to Van (> 64 mcg/ml)– Van B (E. faecium and E. faecalis)

Low level R to Van (4-32 mcg/ml) S to teicoplanin

– Van C (Other Enterococcal species) Low level R to Van and S to teicoplanin Not usually transmitted nosocomially

Vancomycin resistance: modified peptidoglycan precursor VRE surveillance cultures

– Routinely screen admissions to selected ICUs: stool or wound sites

Definitions

MDR – multidrug-R (e.g., “NS” to at least 1 drug in ≥ 3 drug

classes) XDR – extensively drug-R

(e.g., “NS” to almost all classes but retains “S” to at least one drug class)

PDR – pandrug-R (e.g., “R” to all drug classes)

Definitions apply to “acquired” (vs. “intrinsic”) resistance and to drugs that might be used to treat an infection caused by the species.

Specimen: Bronchoalveolar lavage Diagnosis: PneumoniaMany Acinetobacter baumannii

amikacin >32 R amp-sulbactam >32 R

cefepime >32 Rceftazidime>32 Rciprofloxacin >2 Rgentamicin >10 Rimipenem >8 Rmeropenem >8 Rpiper-tazobactam 128/4 Rtobramycin >10 Rtrimeth-sulfa >4/76 R

MIC (µg/ml)

What additional

drugs might we be

asked to test?

Specimen: Brochoalveolar lavage Diagnosis: PneumoniaMany Acinetobacter baumannii

amikacin >32 R amp-sulbactam >32 R

cefepime >32 Rceftazidime>32 Rciprofloxacin >2 Rcolistin 1 Sgentamicin >10 Rimipenem >8 Rmeropenem >8 Rminocycline <1 Spiper-tazobactam 128/4 Rtigecycline 1 *tobramycin >10 Rtrimeth-sulfa >4/76 R

MIC (µg/ml)

“Colistin, minocycline and tigecycline

reported per Dr. Jones request. *No standard interpretive criteria

for tigecycline. Infectious Diseases consult suggested.”

Final Report with Optional

Comment

Cephalosporin's and the like

Major Drugs & Major Activities– Narrow Spectrum: Cefazolin, Cephalexin,

Cefalothin; Staph and Strep except MRSA and enterococci; susceptible Gram-negatives

– Expanded Spectrum: Cefoxitin, Cefotetan; Cefuroxime; Cefaclor; add some Gram-negative and anaerobic coverage.

– Broad Spectrum: Ceftriaxone; Cefixime; Ceftazidime; Cefotaxime; much better Gram-negative coverage; less Gram-positive and anaerobic activity. Ceftazidime has excellent Pseudomonas activity.

– Extended Spectrum: Cefipime: better stability to some of the -lactamases

– MRSA-capable: Ceftaroline, effective vs MRSA, retains decent Gram-negative spectrum.

Resistance Mechanisms: an immense variety of -lactamases; altered Penicillin Binding Proteins (PBP).

Extended-Spectrum β-lactamasesESBL

Variants on enzymes found in Enterobacteriaciae, with two nasty properties: – Have broad activity against cephalosporin's

and semisynthetic penicillin's– May not be detected in vitro by typical

susceptibility testing procedures

Outcome data suggests that they do cause treatment failures in vivo.

The phenotypic definition includes inhibition by clavulanic acid.

ESBLS – What To Do?

Change cephalosporin-class, penicillin-class and aztreonam results to R regardless of in vitro result.

Exceptions:– Cephamycins – not hydrolyzed

cefoxitin, cefotetan, cefmetazole– -lactam inhibitor combinations

The inhibitors do inhibit the ESBLs ticarcillin-clavulanate piperacillin-tazobactam

KPC Carbapenemase’s

Associated with outbreaks of multi-resistant Klebsiella pneumoniae, other Enterobacteriaciae, and even Pseudomonas.

Confers resistance to all -lactams, plasmid-borne

Difficult to detect; 12% of isolates test susceptible to imipenem; ertapenem is most sensitive screening drug, or use screening breakpoint of 2 µg/ml with imipenem or meropenem (most-recent guidelines incorporate this change)

Modified Hodge test used to confirm presence of a carbapenemase (not required if using current breakpoints)

Drugs used to treat these isolates include tigecycline and colistin– Poor clinical outcomes

“Routine” Cumulative antibiogramGenerally…one big report, but

increasing emphasis on segregating data to answer

specific questions

Questions?